The present invention generally relates to radio calling systems, and more particularly to a radio calling system in which a central control station simultaneously broadcasts a pager terminal calling number from a subscriber to a plurality of radio transmitting stations via leased lines and each radio transmitting station simultaneously broadcasts a calling signal at the same radio frequency so as to call the pager terminal.
The pager terminal (bleeper) is becoming very popular, and it is necessary to positively call the user even when the user is at a snowy district or a remote place.
FIG. 1 generally shows an example of a conventional radio calling system for the pager terminal. The radio calling system includes a public line network, a central control station 200 and radio transmitting stations 300a through 300d.
When the subscriber calls the pager terminal via the public line network 100, the central control station 200 simultaneously broadcasts the pager terminal number to the radio transmitting stations 300a through 300d via leased lines. Each of the radio transmitting stations 300a through 300d simultaneously transmit a calling signal of the pager terminal number at the same radio frequency so as to call the pager terminal.
Normally, one region is divided into a plurality of areas. In this example, there are four areas Aa through Ad, and the radio transmitting stations 300a through 300d are respectively located within the areas Aa through Ad. The pager terminal can be called within the entire region by connecting these radio transmitting stations 300a through 300d to the central control station 200 via the leased lines. However, if the leased line connecting to the radio transmitting station 300a fails and the calling data cannot be transmitted to the radio transmitting station 300a, for example, the pager terminal within the area Aa cannot receive the call because the reception sensitivity of the pager terminal is poor. Hence, the reliability of the radio calling system is conventionally improved by providing two systems of leased lines and radio transmitting stations.
FIG. 2 shows the construction of an essential part of the conventional radio calling system for the pager terminal. In FIG. 2, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
In the radio calling system shown in FIG. 2, the central control station 200 includes a data transmitter-receiver 201, and a line switching controller 202. On the other hand, the radio transmitting station 300a includes a data transmitter-receiver 301, a line switching controller 302, a switching circuit 303 for switching the line data, transmitters 304a and 304b which are normally provided to form two systems, a transmitter switching controller 305, a switching circuit 306 for switching the data to the transmitters 304a and 304b, and a transmission antenna 307.
Normally, the communication between the central control station 200 and the radio transmitting station 300a is made via a normal line 500a. Between the central control station 200 and the radio transmitting stations 300a through 300d, inquiries from the central control station 200 and answers from the radio transmitting stations 300a through 300d are carried out at a rate of approximately four times per second. When a call to the pager terminal is generated from the public line network 100, the calling signal is simultaneously broadcast to each of the radio transmitting stations 300a through 300d.
If the normal line 500a fails, for example, the connection between the central control station 200 and the radio transmitting station 300a is switched to an emergency (or spare) line 500b manually by the maintenance person or automatically by the line switching controllers 202 and 302. While the emergency line 500b is in use, the maintenance person restores the normal line 500a, and connection is then switched back to the normal line 500a after the normal line 500a is restored.
However, there are cases where both the normal line 500a and the emergency line 500b fail. In a remote area in the mountains, for example, the lines are carried on telephone poles, and the configuration and cost may make it impossible for the normal and emergency lines 500a and 500b to be carried on different telephone poles. As a result, when the telephone pole which carries both the normal and emergency lines 500a and 500b breaks due to a storm, for example, both the normal and emergency lines 500a and 500b fail.
Conventionally, when both the normal and emergency lines 500a and 500b fail, it no longer is possible to provide the calling service within the area Aa. The chances of both the normal and emergency lines 500a and 500b failing are relatively high particularly if the radio transmitting station 300a is located at the snowy district or the remote place. Moreover, when both the normal and emergency lines 500a and 500b fail, it takes considerable time to restore both lines, and there is a problem in that the calling service within the area Aa is stopped for a considerably long time while the restoration takes place.